The provocative finding that there are 10-fold more corticothalamic (CT) than thalamocortical axons reinforces the long-standing view that the cerebral cortex dynamically regulates its own input via CT neurons. However, the role of corticothalamic circuitry remains a deep mystery. This is because a substantial proportion, perhaps even a majority, of CT cells are silent under standard experimental conditions. Recently, we have found in rat primary somatosensory cortex that weakly responsive CT cells, and even some that are otherwise silent, become more responsive to tactile stimulation of facial whiskers during pharmacologically- induced facilitation of the topographically corresponding area of motor cortex. Thus, inputs from other, functionally related neocortical areas can directly influence the excitability of CT neurons in sensory cortex and hence the processing of afferent, sensory signals in thalamocortical circuits. Understanding the nature of information that is potentially transmitted by these strategically located neurons will likely provide new and important insights into cortical function and its regulation during sensorimotor behaviors. The research plan employs a combination of in vivo and in vitro approaches to examine intrinsic electrophysiological properties, synaptic inputs and receptive fields of CT neurons in the rat somatosensory system. Novel findings will be obtained from two different types of corticothalamic projection systems long-postulated to play distinctly different roles in sensory processing during active touch. The research plan should lead to new insights into how - and perhaps, why - the cerebral cortex regulates its own activity during information processing states. [unreadable] [unreadable] [unreadable]